Interferometer device having a permanently positioned interference pattern viewing screen



` INVENTOR.

1 l I I I I I R. M. DRAKE, JR `INTERF'EROMETER DEVICE HAVING A PERMANENTLY POSITION- INTERFERENCE PATTERN VIEWING SCREEN Filed May 5l, 1949 Oct. 9, 1951 Patented Oct. 9, 1951 INTERFEROMETER DEVICE HAVING A PER- MANENTLY POSITIONED INTERFERENCE PATTERN VIEWING SCREEN Robert M. Drake, Jr., Albany, Calif.

Application May 31, 1949, Serial No. 96,384

1 Claim.

(Cl. {iS-14) (Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) vision is made for recovering light lost from the l last beam splitting plate to produce a visual light interference pattern of the tested medium for continuous use.

Heretofore it has been the usual practice to place ground glass plates in the field of view or 15 image plane of an interferometer device for visual consideration of the interference pattern in making adjustments of the interferometer mechanism and the ground glass plates thereafter replaced by photographic nlm, or the like, for

recording the light phenomena in the test medium. It Iis very'desirable to be able to View the interference pattern at all times without having to disturb photographic film or plates, or

other recording equipment, such that the photographic or recording equipment may be operated at any instant or for any length of time which the operator may choose by reference to the viewing screen.

In accordance with this invention, an interferometer, basically the Mach-Zehnder four-plate system, with or without splitting plates for separating the light rays to produce Schlieren pictures, is modied to the extent that the light rays lost by reflection and transmission from the last glass splitting plate, where the two interference beams are re-united, are recovered by reflection, and concentrated on a ground glass plate `for visual consideration of the interference pattern of the testedA medium. Ordinarily, about twenty per cent of the light from the source was lost or discard-ed as a result of this one reflected and transmitted interference beam at the last glass splitting plate of the interferometer system not being used. In order to increase the light intensity on a ground glass screen of the beam recovered for use, the coatings on the glass splitting plates are apllied to proportion transmitted land reflected light such that the distribution of light is suflcient for all recording and viewing appliances. object the provision of a viewing screen for utilizing the secondary interference beam emerging from the last glass splitting plate of a fourplate interferometer system for visual consider- This invention therefore has for its'50 ation of theinterference pattern falling on the image plane of recording instruments from the primary interference beam.

These and other objects and advantages will become more apparent as the description proceeds when taken in View of the accompanying drawing in which a diagrammatical View shows the relative position of the optical elements of the interferometer system incorporating the interference pattern visual means of this invention.

Referring to the gure of the drawing, there is illustrated a light source l which emits a principal light beam P that passes through a condenser lens 2 Where the beam rays are brought to a focus on a semi-transparent light splitting glass plate 3 set at approximately 45 to the light source and thereafter reflected to a concave front surface mirror 4. The principal light beam P is reflected backwardly in parallel rays from the mirror 4 through the splitting plate 3 onto a second semi-transparent light splitting glass plate 5. Where desirable, a light source may be positioned at the focal point of a lens system producing parallel rays in place of the elements 2, 3 and 4. The splitting plate 5 is disposed at approximately 45 to the principal light beam P from the mirror 4 which splits the light beam P into two partial beams, one partial light beam a being transmitted and the other partial light beam h being reflected. The glass plate 5 is coated such that approximately 26 per cent of the light is reflected for partial beam b and 51 per cent is transmitted for partial beam a as by using a silver or aluminum coating material on the side of the incident light beam P.

The partial light beam a is reflected from a front surface mirror 6 which is set approximately 45 to the incident light beam, as is well known in the Mach-Zehnder interferometer device, to direct the partial light beam a onto a semi-transparent glass splitting plate '1. The glass splitting plate l, also disposed approximately 45 to the incident light beam, is coated with silver, or the like, to transmit approximately 48 per cent and to reflect approximately 48 per c-ent of the partial light beam a. The transmitted partial light beam a passes on to another semi-transparent glass splitting plate 8 that is positioned at approximately 45 to the incident light beam. Like splitting plate l, splitting plate 8 is coated to transmit approximately 48 per cent and to reflect approximately 48 per cent of the partial light beams striking it.

Partial light beam a is rejoined at the splitting plate 3 by the partial light beam b which was reflected by the splitting plate 5. Partial light beam b, after being reflected .by the splitting glass compensating plate Ill, the partial lightV beam b is reflected on a front surface mirror Il set at approximately 45 to the incident partial light beam b and substantially parallel to the mirror 6 to re-unite partial light beams a, and b `at the splitting plate 8. Y The partial light beam b, like the partial light beam a, .is approximately 48 per cent reflected and approximately 48 per cent transmitted by the splitting plate 3.

The reflected partial light` beamA aand the transmitted partial light beam b are re-united at the splitting plate 3 to form the interference beam, referred to V.herein as ab, which strike a front surface mirror l5. The interference beam ab is reflected from the mirror i5 onto the concave front surfacemirror |65v where it is reflected back as a converging beam. At the point of convergence, the linterference beam ab is reflected from a small front surface mirror l1 to the image plane I8 of a camera, or some other recording device, where the interference pattern may be photographed or recorded as is well known in the Mach-Zehnder interferometer system.

That part of the partial light beam a reflected fromV the splitting plate 'l is directed approximately at right angles to the incident partial light beam a andris generally referred to herein as the density light beamY c. VDensity light beam c is reflected from a front surface mirror 2e and again from a front surface concave mirror 2l, similar to the mirror It, to produce a reflected converging density beam c. The converging density beam cgis reflected from the small front surface mirror l1 back to the image plane of the camera i8, recorder, or the like, as known of the Schlieren system for recording air density pictures of the tested medium which tested medium is ordinarily placed between the mirror 6 and the splitting plate 1 and illustrated as a test area I by dashed lines. The medium tested may be a flame in open air, or. a medium Vinclosed, Aas within a windtunnel. Whenever the tested medium includes opticalelements as the windows in a wind tunnel, corresponding optical elements should be placed in the area J in the path of the partial beam b to compensateefor the increase in optical distance through the tested medium optical plates. Some of the mirrors and splitting plates are adjustable to regulate the light path length and the light path angulation as is well known in interferometer systems. The optical distance of the interference beam ab from the last splitting plate to the image plane I8 and of the density beam c fromthe splitting plate 1 to the image plane i8 should be equal as is well recognized in the art.

In order to eliminate the heretofore undesirable practice of rst placing a ground glass plate at the image plane I8 to first View the Schlieren picture and the interference pattern for adjusting pattern foradjusting the interferometer de,-

4 ror 25. The recovered light interference beam d is reected by the mirror 25 and again reflected from a front surface concave mirror 26 into a converging beam that is reflected at its point of convergence or focal point by a small front surface mirror 21 to a ground glass viewing plate 28 at the image plane where the interference pattern may be viewed at all times.

The interference light beam d, heretofore lost or adsorbed,l `amounts to approximately 20 per cent of the primary light beam P at the source. Coatings on the several splitting plates 5, 1, 8 and l0 may be chosen to provide proper light for all the recording and viewing elements. In the present invention, it was found advisable to coat the splitting plate 5 such that it would reflect approximately 26 per cent of light and to transmit approximately 51 per cent of light if metallic coatings are used. The splitting plates 1 and 8 are disclosed herein to both. transmit and reiiect 48 per cent of the light if metallic coatings are used. In this manner, the interference beams ab and d, andV the density beam c are alli of approximately the same light intensity, e'a'ch beam being approximately 2U per cent of the primary light beam P at the source. New nonmetallic coating materials make it possible to vice before placing a photographic plate or other Y to hereinY aS the interference beam d, and ordinarily lost, is recovered by the front surface mirrations than lenses. The concave mirrors I6, 2|V Vand 25 are set with the conjugate axis of each at a slight angle with respect to the incident rays reflected thereon to focus` on Vthe small mirrors l1 and 21, respectively, which are out of the beam paths. c

While I have illustrated a preferred embodiment of my invention, itis to be understood that various modifications and changes-may be made without departing from the spirit and scope of my invention and I desire to be limitedonly by the scope of the appended claim.

I claim: Y Y Y In an interferometer, means for producing a primary beam of collimated light, a partially transparent plane beamsplitting mirrorkincline'd across the primary Vbeam for dividing vthe primary beam into two partial light beams diverging from eachother; plane full mirrors inclined across the partial beams in parallelrelation tothe beam splitting mirror for reflecting ther partial beams across eachother., one pazctial Vbeam being adapted to contain a test 4mediumand constituteia test beam and the other partial beam comprising a comparison beam a second partially transparent beam splitting mirror inclined across the reflected partial beams at the point ofi intersection in parallelrelation to therst partially transparent mirror for passing a portion of each of the partial beams therethrough and reflectingia portion of each of the partial beams'therefrom to form two interference beagms leaving theopposite sides of thepartially transparent mirror in angular relation to each other; Aatlfiwirl. Paftaflly transparent plane mirror inclined across thet'est beam intermediate the test medium and: a second partially transparent; mirror in*parallelA relation to the second'partially transparentmn'ror'forre'- 5 fleeting a portion of the test beam laterally in parallel relation to one of the interference beams to form a Schlieren beam; full plane mirror means in the reflected portion of the test beam and in the Schlieren beam, disposed in parallel relation, for reflecting the last mentioned portions of the test and Schlieren beams in adjacent parallel relation: concave mirrors tilted toward each other in each of the last mentioned parallel reflected portions of the interference and Schlieren beams to dispose their focal points between the last mentioned reflected light beam portions; a plane mirror inclined across the beam portions reflected by the concave mirrors at substantially the concave mirror focal points for reilecting the last mentioned beam portions to a common image plane in adjacent side by side relation; a screen disposed in said image plane for receiving the reilected interference and Schlieren beam portions thereon to form interference and Schlieren images; a plane full mirror inclined across ythe other interference beam to rellect the same laterally; a concave mirror inclined across the said reilected portion of the other interferpoint at one side of the reflected portion of said 6 other interference beam; a plane full mirror inclined across the focal point of the last mentioned concave mirror for reflecting the said other interference beam portion to an image plane; and an image viewing screen in the last mentioned image plane.

ROBERT M. DRAKE, Jn.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,484,584 Tauern et al Feb. 19, 1924 1,957,128 Ball et al May 1J 1934 2,256,855 Zobel Sept. 23, 1941 2,362,235 Barnes Nov. 7, 1944 2,425,758 Saunders Aug. 19, 1947 FOREIGN PATENTS Number Country Date Y 669,880 France Aug. 10, 1929 '720,333 Germany May 1, 1942 234,156 Switzerland Dec. 16, 1944 

